From an observational point of view, various methods can be used to compute the \(c\)  parameter. A first approach to derive the central concentration of DM halos is to measure the gas mass profile in the X-rays out to \(r_{200}\)  or \(r_{500}\) , fit it with an NFW profile and then use the derived \(r_s\)  to compute the concentration. Using this approach, [1]} [2]} [3]}, and [4]} found concentration values higher than expected for their samples of individual FGs
Other authors, like [5]} and [6]}, found normal concentrations in a total of 6 FGs. Again, comparing results with such small statistics and taking into account non-homogeneities in the analytic procedures makes difficult to reach a final conclusion that can be applied to the mean FG population.
